Abstract: The expressions of steady state SO 2 permeation fluxes across pure water and aqueous sodium citrate were derived. The facilitated transport mechanism of SO 2 in the liquid membrane was proposed. The equilibrium approximation and nonequilibrium boundary layer approximation (NEBLA) were used to obtain the permeation flux of SO 2 through membrane solution. The physicochemical and thermodynamical properties of SO 2 liquid membranes system varied with temperature and other factors were considered. Compared with experimental data, the state of NEBLA can better describe the transport behavior of SO 2 than that of equilibrium approximation.